refactor(segments): split runtime modules

[lemtoc]

Issue

Fixes #35

Changes

• Split the runtime segment implementation into focused Rust, Node, Bun, Deno, Python, Nix Shell, and AWS modules.
• Keep shared detection and command collection helpers in the runtime module root.
• Colocate each segment's tests while preserving the existing public exports and behavior.

Verification

• Existing runtime segments keep the same detection and rendering behavior.
• Existing segments::runtime public paths remain available.

Additional Notes

• Local checks passed: rustfmt, Clippy with warnings denied, 269 Rust tests, and the 4-step zsh E2E.
• Local zsh-bench medians: first prompt 28.40 ms, first command 28.58 ms, command lag 0.439 ms, input lag 0.508 ms, exit 20.18 ms.

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[github-actions]

Nova zsh-bench

metric	median	previous	delta	status	green	yellow	orange	red
first prompt lag	30.04 ms	29.45 ms	+0.59 ms	yellow	25 ms	50 ms	100 ms	> 100 ms
first command lag	30.63 ms	29.59 ms	+1.04 ms	green	75 ms	150 ms	300 ms	> 300 ms
command lag	1.01 ms	0.57 ms	+0.44 ms	green	5 ms	10 ms	20 ms	> 20 ms
input lag	0.48 ms	0.72 ms	-0.24 ms	green	10 ms	20 ms	40 ms	> 40 ms

Previous is the stored benchmark for 7a85cb1.
Delta is current median minus previous median. Lower is faster.

Red status fails CI. Values are medians from zsh-bench raw iterations.

be479c4

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